The invention relates to an optical head for scanning the information layer of an optical record carrier. The invention also relates to a lens system for use in such an optical head.
The lens system in an optical head is used to converge a radiation beam from a radiation source to a spot on the information layer of the optical record carrier. A lens in the lens system used as objective lens has generally optical properties that change as a function of temperature, which may cause a deterioration of the quality of the spot. The sensitivity of the lens system to temperature changes can be reduced, i.e. the lens system can be a-thermalised, by arranging a non-periodic phase structure (NPS) in the optical path of the radiation beam as disclosed in the international patent application WO 01/48745 (PHN 17870). Such an NPS introduces a wavefront aberration in the radiation beam as a function of temperature that compensates the wavefront aberration introduced by the lens. However, the NPS makes the lens system more sensitive to wavelength variations in that a change in wavelength introduces spherical aberration in the radiation beam (spherochromatism). Semiconductor lasers may have a significantly different wavelength from batch to batch. For 650 nm lasers commonly used in optical heads this spread may be of the order of 10 nm. Therefore, using an NPS to a-thermalise the lens system may lead to problems of spherochromatism.
Another method to a-thermalise a lens system is to make use of the fact that the wavelength of the semiconductor laser generally changes when temperature changes. A periodic, diffractive structure for converging a radiation beam, such as a grating, is spherochromatic and introduces spherical aberration in the radiation beam passing through the grating when the wavelength of the beam changes. When such a grating is arranged on one of the lens surfaces of a lens in the lens system, it can compensate the spherical aberration introduced by the lens when temperature (and thus the laser wavelength) changes. A drawback of this system is that, when the nominal wavelength varies from batch to batch, an offset in spherochromatism is introduced and the compensation is not adequate anymore.
It is an object of the invention to provide a lens system less insensitive for wavelength variations and at the same time insensitive for temperature variations.
The object is achieved if a lens system according to the invention includes a lens, a grating and a non-periodic phase structure, the lens system being a-thermalised by a compensation of the temperature-dependence of the spherical aberration of the lens by the temperature-dependence of the spherical aberration of the non-periodic phase structure and the grating, and the lens system being a-spherochromatised by a compensation of the wavelength-dependence of the spherical aberration of the grating by the wavelength-dependence of the spherical aberration of the non-periodic phase structure.
The two compensations may be partial or complete. The a-thermalisation is preferably such that the root-mean-square optical path difference (OPD-rms) of the wavefront of a radiation beam passing through the lens system caused by a temperature change of 30 K is less than 20 mxcex, where xcex is the wavelength of the radiation beam. The a-spherochromatisation is preferably such that the OPD-rms caused by a change in wavelength of 10 nm is less than 20 mxcex with refocusing; the a-spherochromatisation is also preferably such that the OPD-rms caused by a change in wavelength of 3 nm is less than 20 mxcex without refocusing.
A preferred embodiment of the lens system is made substantially achromatic. Since the power of the grating is a parameter not fixed by the a-thermalisation or the a-spherochromatisation, the power can be used to make the lens system achromatic.
A further aspect of the invention relates to an optical head for scanning an information layer of an optical record carrier, including a radiation source for generating a radiation beam, a lens system according to the invention for converging the radiation beam on the information layer, and a detection system for converting radiation from the information layer to an electrical detector signal. The a-thermalised and a-spherochromatised lens system reduces the temperature sensitivity of the optical head, making the accuracy of the scanning of the head less dependent on environmental factors and, therefore, more reliable.
A still further aspect of the invention relates to an optical player for scanning an information layer of an optical record carrier, which player includes an optical head according to the invention and an information processing unit for error correction of the detector signal. The improved scanning of the optical head increases the quality of the detector signal and reduces the chances that information read from the record carrier is incorrigible.